The present invention relates generally to an electrical switching device, and more particularly, to a method and apparatus of independently closing contactors of a modular contactor assembly.
Typically, contactors are used in starter applications to switch on/off a load as well as to protect a load, such as a motor, or other electrical devices from current overloading. As such, a typical contactor will have three contact assemblies; a contact assembly for each phase or pole of a three-phase electrical device. Each contact assembly typically includes a pair of stationary contacts and a moveable contact. One stationary contact will be a line side contact and the other stationary contact will be a load side contact. The moveable contact is controlled by an actuating assembly comprising an armature and magnet assembly which is energized by a coil to move the moveable contact to form a bridge between the stationary contacts. When the moveable contact is engaged with both stationary contacts, current is allowed to travel from the power source or line to the load or electrical device. When the moveable contact is separated from the stationary contacts, an open circuit is created and the line and load are electrically isolated from one another.
Generally, a single coil is used to operate a common carrier for all three contact assemblies. As a result, the contactor is constructed such that whenever a fault condition or switch open command is received in any one pole or phase of the three-phase input, all the contact assemblies of the contactor are opened in unison. Similarly, when a closed circuit or conducting condition is desired, all the contacts assemblies are controlled to close in unison. Simply, the contact assemblies are controlled as a group as opposed to being independently controlled.
This contactor construction has some drawbacks, particularly in high power applications. Since there is a contact assembly for each phase of the three-phase input, the contact elements of the contact assembly must be able to withstand high current conditions or risk being weld together under fault (high current) or abnormal switching conditions. The contacts must therefore be fabricated from composite materials that resist welding. These composite materials can be expensive and contribute to increased manufacturing costs of the contactor. Other contactors have been designed with complex biasing mechanisms to regulate “blow open” of the contacts under variable fault conditions, but the biasing mechanisms also add to the complexity and cost of the contactor. Alternately, to improve contact element resistance to welding without implementation of more costly composites can require larger contact elements. Larger contacts provide greater heat sinking and current carrying capacity. Increasing the size of the contact elements, however, requires larger actuating mechanisms, coils, biasing springs, and the like, which all lead to increased product size and increased manufacturing costs.
Additionally, a contactor wherein all the contact assemblies open in unison can result in contact erosion as a result of arcs forming between the contacts during breaking. When all the contact assemblies or sets of contacts are controlled in unison, a detected abnormal condition, such as a fault condition, in any phase of the three-phase input causes all the contact assemblies to break open because the contact assemblies share a bridge or crossbar. Therefore, breaking open of the contacts of one contact assembly causes the contacts of the other contact assemblies to also open. As a result, the contacts may open at non-ideal current conditions. For example, the contactor may be controlled such that a fault condition is detected in the first phase of the three phase input and the contacts of the corresponding assembly are controlled to open when the current in the first phase is at a zero crossing. Since the second and third phases of a three phase input lag the first phase by 120 and 240 degrees, respectively, breaking open of the contacts for the contact assemblies for the second and third phases at the opening of the contacts of the contact assembly of the first phase causes the second and third contact assemblies to open when the current through the contacts is not zero. This non-zero opening can cause arcing between the contact elements of the second and third contact assemblies causing contact erosion that can lead to premature failure of the contactor. This holds true for both abnormal switching as stated above as well as normal duty.
This unison-controlled construction also has disadvantages associated with the closing of contacts to cause high transient current conduction between a power source and a load. The closing in unison of all the contacts can cause mechanical torque oscillations that are often negative. As a result, the windings of a motor as well as the mechanical components of the system are subjected to damaging stresses. Additionally, the motor circuit protection disconnect (breaker or fusing) may have to be oversized to avoid unwanted, or “nuisance”, tripping by this high transient current.
It would therefore be desirable to design a modular electromagnetic contactor assembly having multiple contactors that can be independently controlled such that negative torque oscillations may be minimized. It would be further desirable to design such a modular contactor assembly wherein each contactor is constructed in such a manner as to withstand higher currents under fault conditions without increased contactor complexity and size.